port `sgn` to structured (#58197)
Summary:
https://github.com/pytorch/pytorch/issues/55070
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58197
Reviewed By: ejguan
Differential Revision: D28416538
Pulled By: ezyang
fbshipit-source-id: bd78172ff4b11bfc69304c426d5817a47bcbb567
